Among in-vehicle apparatuses such as a car navigation device, there is an apparatus in which a wireless communication function based on the Bluetooth (a registered trademark: this description will be omitted hereafter) system is mounted. Such a wireless communication function is used for communication with an information terminal, such as a mobile phone or an audio player to realize a hands free call, a dialup connection, playback of music, or the like.
In wireless communications based on the Bluetooth system, the 2.4 GHz band which is called the ISM (Industry Science Medical) band being assigned to industrial use, scientific use and medical use is used. The same 2.4 GHz frequency band is used for wireless communications based on the wireless LAN (Local Area Network) system being compliant with IEEE802.11b/g/n or the like, and cordless telephones, as well as for wireless communication based on the Bluetooth system.
In recent years, a wireless communication function based on the wireless LAN system is mounted on laptop PCs (Personal Computers), smart phones, tablet terminals, etc., and is widely used at home, offices, public wireless LANs, etc. Recently, the number of vehicles mounting such a wireless communication function has been increasing. Then, in a wireless communication function based on the Bluetooth system which is mounted on an in-vehicle apparatus, there arises a problem that communication quality degrades due to the radio wave interference from an ISM band apparatus such as an apparatus using the wireless LAN system. In addition, in the case of an in-vehicle apparatus, there is also a problem that a change of an interference wave channel and a variation in an interference wave level occur in accordance with the moving of the user's vehicle or an adjacent vehicle.
<Specifications of Transmission Power Control Based on Bluetooth System>
Hereafter, the specifications of transmission power control (Power Control) based on the Bluetooth system, which are disclosed in Non patent Literature 1, will be explained.
In the Bluetooth system, the wireless communication device at this end outputs a request to the other wireless communication device being a communication partner based on the received signal strength (RSSI: Received Signal Strength Indication) of a radio wave received by the wireless communication device at this end to control the transmission power of the other wireless communication device so that the RSSI falls within a predetermined signal strength range.
Concretely, (1) when the received signal strength of the wireless communication device at this end is higher than the upper limit of the predetermined signal strength range, a request to decrease the transmission power is sent to the wireless communication device at the other end, and (2) when the received signal strength of the wireless communication device at this end is lower than the lower limit of the predetermined signal strength range, a request to increase the transmission power is sent to the wireless communication device at the other end.
The predetermined signal strength range is defined in the Bluetooth specifications, and is a range of approximately −60 dBm to −40 dBm, currently. More correctly, the lower limit falls within a range of −56 dBm to the actual sensitivity +6 dB, and the upper limit falls within a range higher than the lower limit by 20 dB±6 dB. In this way, according to the specifications, a certain amount of range is permitted for each of the lower and upper limits.
The wireless communication device at the other end which received the request described in the above (1) or (2) decreases or increases the transmission power in accordance with the request.
As a result of the above control performed by both the wireless communication device at this end and the wireless communication device at the other end, the transmission power of each of the wireless communication devices is controlled by the communication partner thereof.
<Problem Regarding Transmission Power Control Based on Bluetooth System>
Because an environment in a vehicle is relatively narrow space, the distance between wireless communication devices is short so that the propagation loss is small. Therefore, in a wireless communication in a vehicle, the received signal strength easily becomes higher than the upper limit of the predetermined signal strength range, and the control tends to be one which decreases the transmission powers of the wireless communication devices communicating with each other.
Further, the fact that the received signal strength tends to be high means that the received signal strength tends not to become lower than the lower limit of the predetermined signal strength range. Therefore, once the transmission power is decreased, the control tends not to become the one in the direction of increasing the transmission power of the wireless communication device.
As a result, the environment in a vehicle tends to be a state in which the received signal strength is decreased. As a result, in an environment in which an interference wave exists, the ratio of the reception power of a desired wave to that of a disturbing wave, i.e., the DUR (Desired to Undesired Signal Ratio) becomes small, and a packet error easily occurs.
As described above, in the specifications of the transmission power control based on the Bluetooth system, the transmission power of a wireless communication device at the other end is controlled in accordance with the received signal strength. Therefore, there is a problem that, even when there is room for improvement in the communication quality by increasing the transmission power, the request to increase the transmission power is not transmitted when the received signal strength falls within the predetermined signal strength range.
In order to solve the above-mentioned problems, for example, Patent Literature 1 discloses a technique of controlling transmission power appropriately in accordance with communication quality, such as a Bit Error Rate (BER) or a Signal to Noise Ratio (SNR).
Further, Patent Literature 2 discloses a technique of controlling the transmission power of the wireless communication device at the other end by sending a request to decrease transmission power to a wireless communication device at the other end periodically, even if the received signal strength of a wireless communication device at the this end falls within a predetermined signal strength range.